Manufacturers of glass sheets (e.g., liquid crystal display, or “LCD”, glass sheets) often heat treat the glass sheets to pre-shrink or compact the glass prior to shipping. Pre-shrinking or compacting glass sheets can be performed at various temperatures that are typically below the glass sheet strain point. Compaction or densification is performed to minimize dimensional changes of the glass during the customer's processing of the glass sheets.
As an example, a glass sheet can be heat treated at 650° C. for a limited period of time (such as approximately one hour) without having an impact on the glass sheet's physical attribute specifications, such as flatness and surface quality. A product that utilizes this process is the Polysilicon Liquid Crystal Display (P-si LCD) device that is used in high-resolution applications. The LCD glass is exposed to relatively high temperatures (e.g., greater than 500° C.) during the manufacturing process. If the glass sheets are not pre-shrunk, the sheets can undergo contour changes that may negatively affect the finished display quality. Pre-shrinking must be performed without creating glass chips that can contaminate the glass surfaces or distort the glass sheet surfaces through spatially non-uniform heating and/or cooling patterns.
Conventionally, a “closed cassette” is used to support glass sheets during heat treatment, such as described in U.S. Patent Publication No. 2005/0193772. An “open” or “standard” cassette is also utilized in some applications. In a closed cassette support method, multiple glass sheets are held in a vertical orientation within enclosed sections of a cassette. The glass sheets are supported with horizontal and vertical supports (such as those made of stainless steel). In practice, the glass sheet is supported around the perimeter so as to maintain the physical attributes such as warp and surface quality. The glass sheet is typically captured along the full length of all four sides.
In an open cassette support method, multiple glass sheets are held in a vertical orientation within a cassette. The glass sheet is supported on the two sides and bottom side with vertical and horizontal supports, respectively. As in the closed cassette support method, the glass sheet is supported around the perimeter to maintain its physical attributes. Both the open and closed cassette methods generally minimize the gravity effect on the glass during heat treatment.
In both the closed and open cassette support designs, the glass sheets are contacted along substantially all of at least three edges. This contact often causes sheet damage or loss. The full-contact supports also have an impact on the thermal characteristics of the system. As may be appreciated, the metal mass concentration along each edge impacts the temperature profile at the edges due to the heat having to travel through metal before reaching the glass along the edges and corners. Additionally, in both support designs, debris (including glass particles and chips) builds up in the bottom-edge support that is very difficult to clean out; as a result, these support designs can cause significant bottom-plate debris contamination of glass sheets.
Both of the aforementioned support designs are manufactured by bending and forming sheet material (such as stainless steel) into the required assembly. By nature, these procedures are not precise, difficult to produce, and costly to manufacture.
Thus, there is a need in the art for systems and methods for supporting glass sheets during heat treatment that maintain the physical geometry and attributes of the glass sheets while minimizing debris contamination and extreme temperature gradients along the edges of the glass sheets.